Levees are as varied in the United States as the people they guard. They're shaped like snakes, rings and spurs that can be tough, flimsy or a century old.

No one knows for sure where all of these earthen walls are, who built them or what type of rocky mixture lies shrouded inside their bulk. Some help protect homes from flooding, while others ring industrial zones containing chemical plants and refineries. Many stop rivers from turning into lakes on farms that abut the riverfront.

They shadow the diverse landscapes they protect. But these widely disparate levees do share at least two common traits -- they are all steadily weakening from water's efforts to go under, over or straight through their earthy embankment. Very few were built to protect against the more powerful storms expected by climate scientists and the flooding that will accompany them.

Now, after years of treating suspect levees with remarkable caution, federal flood officials are recalibrating the way they view the risk to communities lying behind these walls. Government officials say it's driven by better scientific methods used to measure precise flood hazards. But critics believe it could result in still more homes being built in the path of runaway water.

"I do think it's a huge concern," said Shana Udvardy, who directs flood management policy for American Rivers. "The risk is, people are going to believe they're safe when they're not."

At last count, there were roughly 30,000 miles of levees across the country. String them together and they would wrap around the world, and then some. Here's another way to harness their ubiquity: For every McDonald's restaurant in America, there are more than 2 miles of levees.

And most of them, almost 70 percent, are not trusted by government flood officials to do their job. The Federal Emergency Management Agency (FEMA) said in 2010 that 20,350 miles of levee walls are "not accredited" -- meaning that those who live behind the barriers are assumed to be threatened by a "high risk" of flooding.

Which levees are safe?
"without levee." It's a decision based on strict criteria: If a town can't prove that the engineering of its walls is able to repel a flood with a 1 percent chance of occurring each year, then FEMA doesn't consider it to be there.

That's a sober position, many believe, at a time when the arc of development is growing in dangerous flood plains. The "without levee" provision is akin to a red alert. Everyone behind the unaccredited wall is living in a race course for water, FEMA assumes.

That sparks requirements to build homes and businesses that are resilient to flood damage. They might have to be constructed higher, stronger and out of the most dangerous flow ways. Flood insurance is also mandatory behind iffy levees for homeowners with federally guaranteed mortgages; some private banks also demand it.

But taming a flood's potency doesn't come for free. Building to stricter standards adds cost and difficulty to projects that many communities see as vital economic progress. Proposed subdivisions, commercial developments and family homes face higher hurdles than those behind proven levees. In some cases, strips of flood-prone acreage become unavailable -- and less valuable to towns -- for construction.

Treating unaccredited levees as if they've been "completely wiped off the map," as a bipartisan group of 27 senators told FEMA last year, "may be unnecessarily devaluing property and hurting the economies of cities, towns, counties and businesses."

So, now federal flood insurance officials are preparing to scrap the "without levee" approach. Rather than assuming these unproven barriers will do nothing to inhibit the rush of floodwater -- resulting in widespread inundation -- FEMA will begin modeling a variety of dangerous options, which are likely to cause flooding in smaller and more specific areas behind the levee, rather than all over.

"When American jobs are at risk, FEMA should use the methods readily available to it rather than settle for an all-or-nothing approach," the senators said.

Promoting growth in the wrong places?The "without levee" approach will be replaced with a suite of possible scenarios, such as floodwater pouring over the top of some levees, or tunneling through them to cause total collapse, or simply being too low to prevent major inundation in a 1 percent annual flood.

In the end, the policy could enlarge the amount of land behind unaccredited levees that is free from rigorous flood plain regulations, like building standards and mandatory insurance. That could make it easier to build new developments in flood-prone areas, some experts on flooding say.

"It either delays or avoids the really necessary prospect of having more and more people get flood insurance in the flood plain and practice good land use. That's my concern," said Jeffrey Mount, the founding director of the Center for Watershed Sciences at the University of California, Davis. "What bothers me more is not the fact that those people are behind [unaccredited levees]. It's that you're adding people to these places. You're promoting growth in these places."

That's concerning to him and other experts because even accredited levees are bound to buckle eventually. When they do, the damage can be worse than if there were no levee at all, because floodwater can get trapped on the dry side of the wall.

That happened during the landmark Mississippi River flooding of 1993; water broke through agricultural levees before moving along the wrong side of the walled networks until it reached populated areas.

FEMA's plan will rely on improved digital mapping and increased communication with local flood plain officials to determine the risks behind unaccredited levees.

"In many cases, a decertified levee does something," said Eli Lehrer, who heads the Center on Finance, Insurance and Real Estate at the Heartland Institute. "It may not protect against a 100-year flood, but it's foolish to ignore that the levee is in fact there. This does lead to better mapping and better assessment of risk."

The 'funny math' of certificationHe also said the plan could relieve political pressure on FEMA to maintain the accreditation of levees that should not be. That pressure is widespread, as lawmakers and wealthy communities fight to maintain a development-friendly status that comes along with an approved levee. Lehrer said if a portion of the population behind an unaccredited barrier were free from strict flood regulations, the intense lobbying might subside.

"There are a lot of levees that should be decertified but aren't, and everybody knows that," Lehrer said. "If you allow this designation to happen, a lot of the funny math that goes on right now, it'll be a lot easier to stop it if not everybody behind a levee has to buy flood insurance."

But FEMA's plan doesn't address what many experts say is the agency's biggest mistake: the use of the 1 percent annual flood, known as a 100-year flood, as the minimum standard that levees must protect against.

Many experts say the standard should be much higher, like one that requires levees and homes to be built to the 0.02 percent annual standard, commonly called a 500-year storm. Warnings that the current standards are too weak can be traced back to the 1970s, when the Army Corps of Engineers called them "imprudent."

A pioneer of levee policy has been saying the same thing. Gerald Galloway, an expert on flooding at the University of Maryland, is among the strongest voices calling for expanded flood insurance coverage behind levees. The impact of climate change on flooding is one reason for that. There's concern that heavier rainfall combined with increased runoff caused by expanding development is overpowering FEMA's standards.

Flood plains will grow by 45%That was one of the findings by the Interagency Levee Policy Review Committee, which studied the challenges of the nation's levee system following Hurricane Katrina. Galloway chaired the panel.

"The committee recognizes that dealing with climate change, sea-level rise, linked levees, and future development requires the commitment of significant resources and represents a conceptual shift from basing criteria on present conditions to basing criteria on possible, but not fully quantifiable, future conditions," the panel said in its 117-page report. "Nevertheless, the committee believes that this step must be taken."

The agency is not considering the impacts of climate change within its proposed levee plan. But FEMA is studying how flood insurance could be affected by rising temperatures. The study's preliminary findings anticipate that flood plains nationwide will grow by about 45 percent, a huge rise that is expected to increase the program's exposure to risk.

Galloway, for his part, is chairing a new committee to assess FEMA's policy change, so he declined to take a position on the replacement of the "without levee" provision.

But he indicated that his past research, including the findings in a landmark report after the 1993 floods recommending mandatory insurance and limited development behind levees, remains a good way to reduce risk.

"Forget all this business about 'Is it a certified levee or not?'" he said. "No matter how tall your levee is, there's always the possibility that your levee could overtop."